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Method and system of enhanced phase suppression for phase-contrast mr imagingMethod and system of enhanced phase suppression for phase-contrast mr imaging description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20080054900, Method and system of enhanced phase suppression for phase-contrast mr imaging. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATION [0001] The present application is a continuation of and claims priority of U.S. Ser. No. 10/711,603 filed Sep. 28, 2004, the disclosure of which is incorporated herein by reference. BACKGROUND OF THE INVENTION [0002] The present invention relates generally to diagnostic imaging and, more particularly, to a method and system of MR imaging with enhanced flow contrast. The present invention is well-adapted to reconstructing phase-contrast MR images with the suppression of non-zero spatially varying background phase occurring in phase-difference images primarily attributable to eddy currents resulting from the application of magnetic gradients to encode flowing nuclei to static tissue. Moreover, the present invention provides an effective technique to quantitatively assess flow velocities in vivo more accurately. [0003] When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or "longitudinal magnetization", Mz, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated and this signal may be received and processed to form an image. [0004] When utilizing these signals to produce images, magnetic field gradients (G.sub.x, G.sub.y, and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques. [0005] Phase contrast (PC) imaging is an MR acquisition and reconstruction technique that uses flow-encoding gradients with a non-zero first moment to encode spins with a phase proportional to their velocity, .sigma..sub.v=2.pi..gamma.M.sub.1v, where v is the spin velocity, M.sub.1 is the gradient first-moment, and y is the gyromagnetic ratio for protons. The gradient first moment can be calculated from the gradient strength, G, as M.sub.1=.intg..sub.0.sup.TF,G(t)tdt. As such, in a reconstructed PC image, spins flowing at faster velocities may be distinguished from slower flowing spins and from stationary spins. In this regard, PC imaging is commonly used to quantitatively assess flow velocities in vivo. PC imaging is also used to determine the shape of a corpus having flow therethrough. [0006] In MR imaging there are many different sources of phase including differences between the transmit and receive phase, data acquisition window alignment, imperfect slice-selection, filtering, and eddy currents. Typically in PC, to remove these other sources of phase, two separate acquisitions with different first moments and the same zeroth moment (area) are used to remove the phase contribution. [0007] There are two different reconstruction methods for PC, complex-difference (CD) reconstruction and phase-difference (PD) reconstruction. In CD reconstruction, the magnitude of the complex signal difference is calculated from each of the two flow-encoded acquisitions. CD images have zero signal in regions of stationary spins and a non-zero signal proportional to the transverse magnetization and the sine of the velocity-induced phase shift. In PD reconstruction, the difference of the phase from each acquisition is calculated, as .DELTA..sigma..sub.v=arg(Z.sub.lZ.sub.2*), where Z.sub.1 and Z.sub.2 represent the first and second flow-encoded acquisitions, * refers to the complex conjugate, and arg is the phase angle. This is equivalent to .DELTA..sigma..sub.v=2.pi..gamma..DELTA.M.sub.1v, where .DELTA.M.sub.1 is the difference in the first moments between the two acquisitions. The velocity-encoding value or v.sub.enc is defined as the velocity at which spins will be imparted with a phase angle of .pi. radians, or v.sub.enc=1/2.gamma..DELTA.M.sub.1. [0008] Notwithstanding its robustness, phase attributable to eddy currents induced by the flow-encoding gradients themselves is not removed with PD correction. Since these flow-encoding gradients differ between the two separate acquisitions, the eddy currents do not cancel one another when the two separate acquisitions are combined. As a result, the residual eddy currents may manifest themselves as non-zero spatially varying background phase in the PD images. This non-zero background phase is in addition to the phase accumulated by moving spins and can affect the precision of quantitative assessments. [0009] Conventionally, background phase in a PD image is estimated by performing a first-order (or higher) polynomial fit to the image phase. This fit can then be subtracted from the PD image to assess or otherwise remove the background phase. However, this fitting does not distinguish between phase due to actual flowing spins and residual background phase. As such, the determination of the background phase can be effectively over- or under-calculated. In this regard, the presence of phase due to flowing spins can perturb the results of any assessment of the background phase. As a result, quantitative assessment of flow velocities can be affected by the eddy current-induced phase. Accordingly, background phase correction is not usually carried out for quantitative flow protocols. [0010] One proposed solution to correct background phase influences on a PD image is to image a phantom with the same scan parameters used to image the subject. From the phantom scan, background phase is determined and is used to remove the background phase from the PD. While reasonably effective, separately scanning a phantom increases scan time, decreases throughput, and requires additional scanner setup and operator involvement. [0011] Therefore, it would be desirable to have a system and method capable of discriminating between the phase of inflowing spins and the phase of background spins such that background phase correction may be effectively applied for quantitative assessments of flow velocities in vivo. BRIEF DESCRIPTION OF THE INVENTION [0012] The present invention provides a system and method of differentiating between flow induced phase and background phase for correction of PC images that overcome the aforementioned drawbacks. [0013] An automated image processing technique is disclosed that evaluates the pixel phase from a PD image for contributions from flow and non-flow induced sources. From the temporal frequency components of each pixel, a power spectrum of the pixel is generated and evaluated. The power spectrum is evaluated to differentiate between pixels containing predominantly flow induced phase data and pixels containing predominantly background phase data. From this segmentation, the present invention provides a mask image that can be used to subtract background phase from the PD image. Subtraction can be carried out in each direction of flow and, in the context of cardiac or blood flow imaging, for each phase of a cardiac cycle. [0014] Therefore, in accordance with an aspect of the present invention, an MR scanner includes an MRI system having a plurality of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field and an RF transceiver system. The MR scanner also includes a computer programmed to generate PD images from a first acquisition and a second acquisition and determine a non-zero spatially varying background phase from the PD images that are due to eddy currents induced by flow encoding gradients used to generate the PD images. The computer is also programmed to remove the non-zero background phase from the PD images and determine phase associated with flowing spins and phase associated with stationary spins. [0015] In accordance with another aspect, the present invention includes a method of phase correction in flow analysis MR imaging. The method includes the step of determining temporal frequency components for a given pixel of a phase-difference image. The method also includes generating a power spectrum of the given pixel from the temporal frequency components and determining a percentage of the power spectrum within a given energy range. The method also includes the step of including the pixel in a mask image if a given percentage of the power spectrum for the pixel is at a given energy. In an alternate aspect, selected harmonics of the power spectrum of the given pixel are summed and normalized to a maximum pixel value in the phase-difference image. From this normalization, background pixels are distinguished from flow pixels. [0016] In accordance with another aspect, the invention is embodied in a computer program stored on a computer readable storage medium and having instructions which, when executed by a computer, cause the computer to determine a pulsatility factor for each pixel of a slice of a PD image. The computer is also caused to generate a mask image of the slice from only those pixels having a pulsatility factor below some nominal value and apply the mask image to the phase-difference image for the slice to mask the PD image along the slice. The set of instructions further causes the computer to reiterate these acts for each slice of the PD image and output a corrected PD image with substantial subtraction of background phase. [0017] Various other features, objects, and advantages of the present invention will be made apparent from the following detailed description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS [0018] The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention. [0019] In the drawings: [0020] FIG. 1 is a schematic block diagram of an MR imaging system for use with the present invention. Continue reading about Method and system of enhanced phase suppression for phase-contrast mr imaging... Full patent description for Method and system of enhanced phase suppression for phase-contrast mr imaging Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Method and system of enhanced phase suppression for phase-contrast mr imaging patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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